The main attraction of St. Johns County is the school system, full stop. After that, the attraction is that it checks off a lot of boxes pretty well that people who prefer living in bedroom suburbs like, and you have access to Jacksonville without living in Jacksonville (and thereby contributing taxes toward the assets you benefit from). It has cachet as the go-to spot for suburbanite families, but this is of course subjective.
Many other points are just confirmation bias for people who are already attracted based on those things. It's got low taxes (well, sort of, but you're paying a lot on impact fees); it's convenient to Jacksonville and it's employment centers, shopping, and things that can't be found in generic bedroom suburbs (it's much farther than comparable areas like Mandarin and Southside, let alone closer neighborhoods); it's "safe" (the crime rate is no different than that in similar areas across the metro area and country, and it's less safe if you're trying to use the roads, especially for walking or biking) it's close to the beach (most of the county is not, and even Ponte Vedra has among the worst beach access in Florida); you get a lot of land for your money (you don't, looking at comparable areas); it's close to St. Augustine (most of the developments are literally as far as you can get from St. Augustine and still be in the same county); it's good for investment (maybe in the short term, definitely not in the long term).